Sizing Conductors for VFDs, Defining Terms and More

If you have a problem related to the National Electrical Code (NEC), are experiencing difficulty understanding a Code requirement, or are wondering why or if such a requirement exists, ask Charlie, and he will let the Code decide. Questions and comments can be sent to codefaqs@earthlink.net.

Sizing conductors for VFDsWe have a motor control center (MCC) where we removed the combo starters and landed the wire directly to the circuit breakers and then to a variable frequency drive (VFD). These breakers are all inverse-time breakers, and in referencing the Code, I am using Sections 240.6, 430.52(C)(1) and Table 430.52 to confirm that no breaker or wire size is over the 250-percent rule for inverse-time breakers. Here is an example: from a 70-ampere (A) breaker in the MCC, 8 AWG wire was run to a disconnecting means where we have now installed a VFD. The motor at full load does not draw more than 37.8A at 480 volts (V). We are being told that we are in violation of Section 430.122 and that all wiring from the MCC to the VFD needs to be replaced. What is the correct answer?
The full-load current of the motor must be taken from Table 430.250. You didn’t give the motor horsepower, but I am assuming it is 30 horsepower from the 37.8 full-load amperes you present. The full-load current from Table 430.250 for a 30 horsepower, 460V motor is 40A. Using Table 430.52, the maximum rating permitted for the branch-circuit short circuit and ground-fault protective device using an inverse-time breaker is 250 percent, which allows a 100A rated overcurrent device (40 2.50 = 100). The size of the motor-circuit conductors is determined by 430.122, which requires an ampacity rating of 125 percent of the motor full-load current taken from Table 430.250. The ampacity of the branch-circuit conductors is required to be 40 1.25 = 50A. This permits the use of 8 AWG conductors.

Defining terminologyI hear the terminology adjustable speed drives (ASD) and variable speed drives (VSD) used. But what is a VFD?
ASDs and VSDs are the same and refer to controllers that vary the speed of AC or DC electric motors. A VFD refers to alternating current (AC) electric motors only. A VFD flucuates the speed of an AC motor by varying the frequency to the motor. A VSD controlling a direct current (DC) motor alters the speed by changing the voltage to the motor.

Confused terminologyIs an electric motor soft-start starter the same as a VSD?
No. With a soft-start starter, the motor is started with reduced voltage. When the timing circuit is run out, a bypass contactor pulls in, and the motor continues to run at full speed. With a VSD, you have continuous control of the motor speed, and you can change the speed by varying the output frequency from the drive unit.

Designating area under raised floorsWhy isn’t the area under a raised floor in an information technology equipment (ITE) room considered an air-handling area subject to the requirements of Section 300.22(C)?
NEC 300.22(D) permits electrical wiring in air-handling areas beneath raised floors for ITE to be installed in accordance with Article 645. Article 645(D) contains requirements for installing supply circuits and interconnecting cables in the environmental air space beneath the raised floor of an ITE room that are less stringent than the requirements shown in 300.22(C) based on compliance with all of the provisions of 645.4.

Cord length requirementsI would like to know the Code on the maximum length of cord that can be installed on equipment to be plugged into an outlet. I am working at a research center, and they are installing eight electronic core switches. The power required is 8–20A, 220V, L6 twistlock outlets in a computer room with a raised floor. The existing outlets are under the raised floor 25 feet away from the rack. The switches are being installed in the existing rack above the floor with 20- and 25-foot cords run under the raised floor to existing outlets. Is this Code-compliant?
The electronic core switches are considered part of the data processing equipment. NEC 645.5(D)(3) permits supply cords of listed information technology equipment under raised floors in accordance with 645.5(B)(1), which limits the length of the flexible cord and attachment plug cap to not exceed 15 feet.

Current carrying clarificationRegarding counting the neutral as current-carrying in a multiwire branch circuit, I’m not clear if your answer in your June column was that you must count it under certain conditions or don’t count it at all. In residential work, the loads on each hot leg of multiwire branch circuit change.
According to 310.15(B)(4)(a), in a 3-wire circuit consisting of two-phase conductors and the neutral of a single-phase system, the neutral conductor carries only the unbalanced current from other conductors of the same circuit and is not required to be counted when applying the provisions of 310.15(B)(2)(a).

Neutral-service conductorCan the neutral-service conductor be run as a bare (uninsulated) conductor in a metal raceway?
A grounded (neutral) copper service-entrance conductor is permitted to be run uninsulated in a raceway [230.41 Exception (1)]. Note that this is permitted for copper conductors only.

Mounting disconnect behind AC unitWe are having a debate whether an air conditioning unit nonfused disconnect can be mounted behind the air conditioning unit, or does the working clearance have to be met?
NEC 440.14 requires the disconnecting means to be readily accessible. Readily accessible means capable of being reached quickly for operation without needing to climb over obstacles and so forth. NEC 110.26 requires sufficient access and working space about all electrical equipment to permit ready and safe operation and maintenance of such equipment.

Fastening Romex cable requirementsIs it required that each Romex cable be fastened to the panel with a Romex connector? Why can’t I pull several cables into the panel through a 2-in. bushing?
NEC 312.5(C) requires that each cable shall be secured to the panelboard. However, there is an exception that permits nonmetallic-sheathed cables to enter the top of a surface-mounted panelboard through nonflexible raceways not less than 18 inches and not more than 10 feet in length provided conditions (a) through (g) are met:

“(a) Each cable is fastened within 12 in., measured along the sheath, of the outer end of the raceway.”

This means the cable must be fastened within 12 inches of its entry into the raceway.

“(b) The raceway extends directly above the enclosure (panel) and does not penetrate a structural ceiling.”

This means the surface-mounted panel will have a surface-mounted nonflexible raceway extending out of the top of the panel and extending not less than 18 inches and not more than 10 feet above the panel and not penetrating a structural (drywall) ceiling. The raceway is permitted to penetrate a nonstructural ceiling such as a lay-in type ceiling. This is to provide easy access to the outer end of the raceway.

“(c) A fitting is provided on each end of the raceway to protect the cables from abrasion and the fittings remain accessible after installation.”

This requires a bushing on each end of the raceway.

“(d) The raceway is sealed or plugged at the outer end using an approved means so as to prevent access to the enclosure (panel) through the raceway.”
This will prevent foreign materials or objects from entering the panel through the raceway.

“(e) The cable sheath is continuous through the raceway and extends into the enclosure (panel) beyond the fitting not less than in.”

This will provide protection for the cable conductors from any future work installing additional cables into the raceway that may be done.

“(f) The raceway is fastened at its outer end and at other points in accordance with the applicable raceway article.”

Securing and fastening the raceway is required according to the applicable raceway article.

“(g) Where installed as conduit or tubing, the allowable cable fill does not exceed that permitted for complete conduit or tubing systems by Table 1 of Chapter 9 of this Code and all applicable notes thereto.”

Table 1 generally applies to complete conduit or tubing systems but is expanded to apply to this use.

Hot wire and neutral disconnectsWhere in the 2008 Code does it say three hots with one neutral must be on a three-pole, single-throw disconnect?
NEC 210.4(B) requires that each multiwire circuit be provided with a means that will simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates. This can be accomplished by using a multipole circuit breaker or by using approved handle ties. Be sure to note 210.4(D), which requires grouping of the ungrounded and grounded conductors of each multiwire branch circuit in at least one location within the panelboard.